The Obama administration sent a team of special forces to Syria this summer to rescue American hostages held captive by Islamic militants, but the operation failed, U.S. officials said Wednesday.

"The U.S. government had what we believed was sufficient intelligence, and when the opportunity presented itself, the president authorized the Department of Defense to move aggressively to recover our citizens," Lisa Monaco, a top counterterrorism adviser to President Obama, said in a statement. "Unfortunately, that mission was ultimately not successful because the hostages were not present."

Monaco's statement did not name the hostages targeted by the operation or the precise timing, saying only that the attempt occurred "earlier this summer." But an administration official said they included slain American journalist James Foley.

The hostages were held by the militant group now known as Islamic State and were under increasing danger, the statement said.

The unusual acknowledgment of a failed secret mission came the day after a video surfaced online showing Foley's beheading. A masked fighter in the video also threatened to kill another American, Steven Joel Sotloff, who is believed to be held captive by the group.

Foley's death came despite repeated pleas from his family, friends and employer to find the 40-year-old New Hampshire man and return him to safety.

"Our thoughts and prayers are with the remaining hostages' families and their loved ones during this difficult time," Monaco said. "We continue to call for their immediate release. On behalf of all Americans."

The Pentagon said the unsuccessful rescue attempt involved air and ground components.

"As we have said repeatedly, the United States government is committed to the safety and well-being of its citizens, particularly those suffering in captivity," Pentagon spokesman John Kirby said in a statement. "In this case, we put the best of the United States military in harm's way to try and bring our citizens home."